As is well known in the art, it is desirable to fabricate many different types of semiconductor devices in very thin uniform layers of epitaxial GaAs. These devices include solar cells, field effect transistors, and various types of known GaAs microwave devices. In the fabrication of certain types of high frequency GaAs field effect transistors, for example, it is preferred to form the active transistor device regions in a very thin epitaxial layer of semi-insulating GaAs on the order of about 0.2 micrometers or less in thickness. One liquid phase epitaxial (LPE) process for forming such semi-insulating epitaxial GaAs layers is described in a copending application Ser. No. 530,336, assigned to the present assignee. Therefore, the desirability and preference to fabricate thin high quality epitaxial layers of GaAs under controlled conditions and at high yields are manifest.